The present invention relates to a monitoring device for monitoring an inductive energy transmission device.
Electric vehicles and hybrid vehicles usually have an electric energy store, for example a traction battery, which makes electric energy available for the drive. If this electric energy store is wholly or partially discharged, the electric vehicle must drive to a charging station at which the energy store can be charged up again. It has been usual until now for this purpose that at such a charging station the electric vehicle is connected to the charging station by means of a cable connection. This connection must usually be established manually by a user. It is also required here that the charging station and the electric vehicle have a connecting system corresponding to one another.
A few cable-free charging systems for electric vehicles or hybrid vehicles are also known. An electric vehicle is parked for this purpose over a transmitting coil (transmitting device), or a charging pad or charging device. This coil transmits a high-frequency alternating magnetic field. The alternating magnetic field is received by a receiving coil (charging coil or receiving device) inside the vehicle and converted into electric energy. A traction battery of the vehicle can thereupon be charged by means of this electric energy. Document DE 10 2011 010 049 A1 discloses such a system for charging a vehicle battery in which the energy is transferred inductively.
The energy store of the electric vehicle can, furthermore, also be used for energy recovery. A cable connection or also an inductive energy or power transfer can also be used for this purpose.
For the cable-free charging of a battery of an electric vehicle, the transmitting coil of the transformer typically is either inset into the road surface or is formed as a charging plate (charging pad) placed on the ground, and is connected by means of a suitable electronic system to the electric grid. The receiving coil of the transformer is typically mounted permanently into the floor of the vehicle and for its part is connected by means of a suitable electronic system to the traction battery of the vehicle. For energy transfer, the transmitting coil or primary coil generates a high-frequency alternating field which penetrates the receiving coil or secondary coil where it induces a corresponding current. Since on the one hand the transmitted power scales up linearly with the switching frequency, and on the other hand the switching frequency is limited by the drive electronics and by losses in the transmission path, a typical frequency range of between 30 and 150 kHz results.
There is an air gap between the transmitting coil of the charging station and the receiving coil in the vehicle. Due to the ground clearance necessary for motor vehicles, this air gap amounts to a few centimeters. Air gaps with a size of between 3 and 30 cm are thus very widespread whenever an ideally small air gap is not achieved through measures such as lowering the coil that is fixed to the vehicle, the entire vehicle, or raising the locally fixed coil, or a combination of these measures. The alternating magnetic fields arising in the air gap during the transmission are capable of inducing electric eddy currents in any metal or electrically conductive objects that are located in the air gap. These so-called foreign objects heat up due to ohmic losses. This heating represents a significant danger, not only for personal safety, but also for the operational reliability of the vehicle. It is therefore necessary either to limit the heating of an inductive charging system by restricting the magnetic field, or to detect any objects that may be located in the air gap using suitable means and thereupon to deactivate the energy transfer until these are removed or until they no longer represent a danger.
Known methods for the detection of foreign objects consist, for example, in conventional inductive metal detection using additional sampling coils which are subjected to pulsed excitation and whose electrical decay behaviors are analyzed. A version of a coil array which detects the presence of the foreign objects using its own magnetic field by means of eddy current losses or through a shift in the coupling factor, or through the variation of the coil quality is particularly appropriate here. The diameter of the coil plays a crucial role here. The smaller the coil diameter, the more precise is the resolution of position, and the more possible it is for small objects (cent coins, paperclips, etc.) to be detected. It is disadvantageous that a fine-mesh array does not illuminate the whole of the intermediate space between the transmitting coil lying on/in the ground and the receiving/charging coil located in the vehicle. The reason for this is that the magnetic field of the sensor array/coil array only illuminates/detects approximately as high as the length of the diameter of the coil. A fine, close-mesh network/array thus does not detect any metal objects that are arranged high above the ground coil (an upright yoghurt container with aluminum lid, cigarette pack, metal bars that are poking in etc. for example).
There is therefore a need for a monitoring device for inductive energy transmission devices by means of which the detection height of the coil array used for detection can be made scalable without thereby changing the mechanical configuration of coils.